Civilian and military sea vessels use both active and passive sonar systems for numerous purposes including geological studies, marine life exploration, and military operations such as anti-submarine warfare (ASW). These systems are used to detect the presence of submerged objects by either transmitting a sound wave and detecting its reflection as it propagates through the water (active sonar) or by listening for sound waves generated by these objects (passive sonar).
These systems generally utilize sonar transducers, for example, hydrophones in conjunction with sound receiving and signal processing electronics to detect the presence of a submerged object. A database of known sonic frequencies is often used to aid in the target identification process. For example, in the case of military operations, vessels may be identified by the unique noises generated by their propulsion systems, or by the distinct operating frequencies of their AC power systems.
The use of passive sonar systems may be advantageous over active systems, as passive systems are “silent” in operation. Specifically, a host vessel's location is not reveled by the use of passive sonar systems, whereas the transmission of a sound wave (a “ping”) by an active sonar system, while potentially providing range and bearing information of a target, also greatly increases the ability of other vessels to detect these pings, and thus the presence and/or location of a searching vessel. Accordingly, passive sonar is particularly useful in military operations, such as ASW, where undetected operation is of critical importance.
A drawback of passive sonar, however, is that it is subject to interference, particularly by noise emitted from the host vessel. For example, noise from the vessel's propulsion system may negatively impact the operation of a passive system. This is especially true in the case of hull-mounted arrays, where hull-born vibrations and other noises are transferred directly to the sonar transducers. In order to locate the array further from the vessel's noise-producing components, and thus reduce interference, sonar arrays are often towed behind vessels.
These towed arrays generally comprise hydrophone arrangements that are typically deployed and recovered through openings in the hull of the vessel below the water line. Thus, if the vessel is not originally constructed with this type of arrangement, the vessel must be dry docked and a portion of the hull removed in order to be retrofitted with the system. In the case of larger arrays, or cases where a hull-opening is not a preferred option, the towed array is often deployed and recovered over the gunwale of the ship using, for example, a winch and boom arrangement. This handling equipment occupies a large amount of deck space in addition to presenting a large target cross-section to enemy radar. Thus, these arrangements limit covert deployment and recovery. Deployment and recovery is also time consuming, expensive, and difficult in high seas.
Moreover, towed arrays can often not be deployed in shallow waters. This is an increasing problem as military and civilian vessels are beginning to operation in these more littoral waters. Towed arrays also negatively impact the maneuverability of a vessel, hindering, for example, “sprint-drift” and “sprint-stop” detection maneuvers, as well as the speed and mobility of a vessel in general when towing the array. Towed arrays are subject to various types of flow-noise, such as turbulent boundary layer (TBL) noise, extensional wave noise generated by strengthening or stiffening members typically utilized in the towed arrays, and breathing wave noise produced by the movement of fluid or flexible fill used within the hydrophone arrangements.
In the case of hull-mounted arrays, the same types of installation challenges exist. Specifically, installing these systems typically involves disposing a small sonar array on the hull of a vessel, such as on the bow, which normally requires cutting out a section of the hull and reinforcing it to maintain adequate seaworthiness. Accordingly, unless originally fitted to the vessel, these procedures require dry-docking and significant time out of service. As these installations are extremely expensive, they are not often performed on small vessels, including small non-military vessels.
Accordingly, it would be advantageous to have a hull, or surface-mounted passive sonar system which can easily and cost-effectively be retrofitted to existing vessels of all sizes, for both military and civilian uses. It would also be advantageous to provide a hull-mounted sonar system which does not hinder the operation of the vessel during, for example, tight maneuvering, especially in shallow waters.